An illicit relationship would be some bs forbidden one MOST likely by Muslim faith or Middle Eastern culture, which is non sense. Like mixing a Muslim with a Christian, they would lose it if that happened in the Middle East. Or a normal blood person marrying a royal blood person. However a boyfriend/girlfriend relationship is usually done by choice of both parties wanting to spend time and be with the other with no restrictions or secrecy in hopes of NOT disappointing their parents or religion.
Illicit relationship is the one that law doesn’t allow.
Before marriage, there is no law to guide/resist your romantic involvements. So even if you are in a relationship that is unethical – i.e like having a girlfriend and still sleeping with another girl – that is still lawful.
After marriage, you are bound to a person by law. You are not supposed to have other romantic engagements (atleast as per law of India). Hence if you pursue such a relationship after marriage, with someone other than your husband/wife, that is illicit and is unlawful.